1. Field of the Invention
The present invention relates to a method and fluid additive for use in treating subterranean formations containing clays and particularly for use in stimulation fluids such as fracturing fluids.
2. Description Of The Prior Art
Hydraulic fracturing is commonly employed to increase the production of fluids from a subterranean formation. Hydraulic fracturing involves the injection of a suitable fracturing fluid down a well penetrating a formation and into the formation under sufficient pressure to create a crack or fracture in the formation. The created crack or passageway facilitates the flow of fluids through the formation and into the well.
In a typical fracturing treatment a particulate propping agent such as sand, sintered bauxite, glass beads, alumina, resin coated sand or the like is introduced into the created fracture to assist in keeping the fracture at least partially propped in an open position. The propped open passageway further enhances the flow of formation fluids. To facilitate transport of the proppant material into the created fractures, the fracturing fluid typically is viscosified with one or more of the various well known polysaccharide gelling agents. The gelling agent imparts viscosity to the fluid thereby permitting enhanced proppant transport and reduces fluid loss from the fracturing fluid to the formation. As the gelled fracturing fluid is introduced into contact with the subterranean formation, a portion of the fluid leaks off into the formation. As the fluid leaks off into the formation matrix, the gelling agent is deposited on the face of the created fracture and is concentrated thereon by the action of filtration as additional fluid leaks-off. The gelling agent often is admixed with a crosslinking agent to further enhance the viscosity of the fracturing fluid.
Combination fracture-acidizing processes also are well known in the art. In such processes an aqueous acid is admixed with a suitable viscosifier and utilized in the formation treatment resulting in enhanced formation permeability.
Upon conclusion of the fracturing treatment, the fracture is permitted to close upon the propping agent and at least a portion of the treatment fluid generally is flowed back to the surface for removal. To facilitate return of the viscosified fluid, additives commonly referred to as "breakers" are included in the fluid by reaction with the polysaccharide gelling agent which is present in the fluid.
It has been found that when the subterranean formation contains clays, such as smectite, illite, kaolinite, chlorite and mixed layer clays and the like, that the gelling agent can be adsorbed by the clays during the treatment of the formation and result in a significant reduction in formation permeability. The polysaccharide gelling agent adsorbs on the clay platelets between the layers, swelling the clay structure and reducing the permeability of the formation.
It would be desirable to provide a method by which the adsorbed polysaccharides can be removed from the clays to thereby increase the formation permeability.